Carbohydrate Dependence During Prolonged, Intense Endurance Exercise
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REVIEW ARTICLE
Carbohydrate Dependence During Prolonged, Intense Endurance Exercise John A. Hawley1,2 • Jill J. Leckey1
Ó The Author(s) 2015. This article is published with open access at Springerlink.com
Abstract A major goal of training to improve the performance of prolonged, continuous, endurance events lasting up to 3 h is to promote a range of physiological and metabolic adaptations that permit an athlete to work at both higher absolute and relative power outputs/speeds and delay the onset of fatigue (i.e., a decline in exercise intensity). To meet these goals, competitive endurance athletes undertake a prodigious volume of training, with a large proportion performed at intensities that are close to or faster than race pace and highly dependent on carbohydrate (CHO)-based fuels to sustain rates of muscle energy production [i.e., match rates of adenosine triphosphate (ATP) hydrolysis with rates of resynthesis]. Consequently, to sustain muscle energy reserves and meet the daily demands of training sessions, competitive athletes freely select CHO-rich diets. Despite renewed interest in high-fat, lowCHO diets for endurance sport, fat-rich diets do not improve training capacity or performance, but directly impair rates of muscle glycogenolysis and energy flux, limiting high-intensity ATP production. When highly trained athletes compete in endurance events lasting up to 3 h, CHO-, not fat-based fuels are the predominant fuel for the working muscles and CHO, not fat, availability becomes rate limiting for performance.
& John A. Hawley [email protected] 1
The Mary MacKillop Institute for Health Research, Centre for Exercise and Nutrition, Australian Catholic University, Locked Bag 4115, Fitzroy, VIC 3065, Australia
2
Research Institute for Sport and Exercise Sciences, Liverpool John Moores University, Liverpool, UK
1 Introduction and Background The major metabolic consequences of the adaptations of skeletal muscle to endurance training are a slower utilization of carbohydrate (CHO)-based fuels (muscle and liver glycogen, blood glucose and muscle, blood and liver lactate), a greater reliance on fat-based fuels [adipose and intramuscular triglycerides (TGs), blood-borne free fatty acids (FFAs) and TGs] and less lactate production during low- to moderate-intensity exercise [i.e., 45–65 % of maximal oxygen uptake (VO2max)]. These adaptations, in part, underpin the substantial increases in submaximal exercise capacity observed following endurance training. Accordingly, many athletes and coaches steadfastly believe that fat plays an important role while training for and competing in endurance events lasting up to 3 h and that training and/or nutritional strategies that ‘‘spare’’ CHObased fuels and enhance the oxidation of fat-based fuels will improve exercise capacity. In this review, we demonstrate that rates of fat oxidation over a wide range of speeds/power outputs are not substantially altered after endurance training when exercise is undertaken at the same relative intensity. This is because a major goal
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